The present invention relates generally to chemical detection systems for detecting trace amounts of chemicals, e.g., explosives or narcotics, on clothes, baggage, vehicles, shipping containers, etc. Detectors used in trace explosives detection systems include ion mobility spectrometers (IMS), mass spectrometers (MS), surface acoustic wave sensors (SAW), electron capture devices (ECD), differential mobility spectrometers (DMS), and chemiluminescence detectors (CLD).
In general, IMS detectors (and others with similar operating principles) excel at detecting very small amounts of explosives; including low vapor pressure explosives such as TNT, RDX, PETN, and HMX. Recently, IMS detectors have been successfully miniaturized into a lightweight, battery-powered, hand-portable unit (such as disclosed U.S. Pat. No. 6,978,657, which is incorporated herein by reference).
Two different methods are commonly used to collect samples. Particles of contraband (e.g., explosives, narcotics) are typically collected by swiping a contaminated surface with a small piece of cotton cloth or flexible metallic screen. Vapors (and particles) of contraband can be collected and pre-concentrated by pulling (i.e., vacuuming) contaminated air through a porous metallic screen (such as a stainless steel screen, felt, or screen). Low vapor pressure explosive molecules are “sticky”, meaning that they easily adsorb onto the metallic screen. However, high vapor pressure explosives pass through the preconcentrator screen without sticking. Short, concentrated puffs of air can be directed to a surface to dislodge contaminants stuck in clothing, etc., which are then sucked into the preconcentrator module.
Next, the contaminated screen can be removed from the preconcentrator module and placed in a thermal desorption chamber located close to (or, as part of) the detector. In the desorption chamber, the metallic screen is heated to about 180 C to 220 C to vaporize and desorb the contaminants. Depending on how fast the screen is heated up, the contaminants may be released slowly or quickly. Typically, the screen is rapidly heated in a single pulse from room temperature to about 200-210 C in a very short time period (e.g., 0.2-0.4 seconds). With this type of heating pulse (i.e., flash heating), almost all of the collected particles and adsorbed vapors are rapidly vaporized and released at essentially the same time; thereby producing a single, concentrated pulse (i.e., packet, bunch, or group) of analyte gas. Then, a carrier gas (e.g., clean, dry air or nitrogen), flowing through (or across) the screen, carries the desorbed contaminants to the detector, such as a miniature ion mobility spectrometer (IMS) or miniature mass spectrometer (MS). This mode of desorption (flash) generates a much higher concentration of analyte gas, as compared to continuous air sampling. Hence, by flash-desorbing a preconcentrator screen, the signal-to-noise (S/N) ratio of the detector can be increased by a factor of 1000 times or more; as compared to systems that don't use a preconcentrator screen.
The preconcentrator screen (also called a preconcentrator mesh or substrate) is typically heated by flowing a high-amperage (e.g., 50-100 amps) electric current across a stainless-steel screen from one edge of the screen to an opposite edge of the screen; thereby generating internal heat energy by Joule-type electric resistance heating (Pheat=I2R). For example, a 12-volt gel-cell type battery can provide about 60 amps of current through a screen having an in-plane electrical resistance, Rs, of about 0.2 ohms for a short pulse (e.g., 0.3-0.5 seconds); which generates sufficient heat energy to raise the screen's temperature from room temperature to about 200 C The low impedance of the screen (e.g., 0.2 ohms) effectively short-circuits the battery, causing a high current to flow through the screen. We have found that only certain types of batteries are suitable to provide such a high current draw (e.g., a lead-acid car battery).
In battery-powered, hand-portable detector systems, the battery's voltage gradually decreases (i.e., discharges) over time due to regular use, aging, etc. Hence, assuming a fixed heating time period and a fixed screen electrical resistance, the peak screen temperature also decreases as the battery's voltage decreases. Variances in peak screen temperatures, Tmax, can occur due to variations in the geometry, resistance, contact oxidation, etc. from one screen to the next; or due to variations in properties from one battery to the next.
Since the rates of thermal desorption for the adsorbed contraband chemicals depend strongly (i.e., exponentially) on temperature, then small variations in the peak screen temperature can cause unwanted, large variations in the amount of contaminants desorbed from the screen. What is needed, then, is a heating control system and a control methodology that can provide accurate, repeatable and consistent peak screen temperatures, despite these variations and uncertainties. In particular, a needs exists for compensating for the gradual decline of the battery voltage; as well as for compensating for screen-to-screen variations and battery-to-battery variations. Against this background, the present invention was developed.